My friends computer has an issue where it would randomly lock up. The entire system just stops responding. Mouse freezes too. Seems it happens more often when playing civ 5. Need to track down why. Currently we think it's due to faulty RAM, so we've got memtest running.

Any ideas?

Additional info:
Sometimes when it freezes, The mouse still works for a lil while then stops. Also it seems the USB devices lose power at this point.
No crash dump is generated.
CPU is core i3, 8GB corsair XMS3, H61 MSI MoBo, HD4870 graphics, Antec PSU.
Only a few months old.
Has a new WD HDD and an old SeaGate HDD as secondary.

Memtest turned up no errors.
Disk check found some errors and corrected them.
PSU is a 620W Antec, so power droops shouldn't cause that much instability.

Gonna try disconnecting the old HDD. See if that makes it more stable.

EDIT: just crashed when trying to set up a civ 5 game. USB devices all lost power and the headphones started making very weird noises. Kinda like electronic noises, not static, not buzzing, kinda like data. It's weird, and I've never heard of it happening before.

As an update, Scan tested the MoBo and found "no fault". Going to replace the PSU next, but really... I've never seen a brand new PSU from a well known brand like Antec to cause so many problems before. After a couple years maybe, but not brand new.